Monterey’s kelp forest puzzle: why sea otters leave some urchins uneaten

Look up, then follow a kelp stem down

Look upward through a kelp forest and the habitat continues far above your head. Blades catch the light, small gas-filled floats lift them, and long stipes run down toward holdfasts anchored to rock. Giant kelp is a brown alga, but the word forest captures something a diver immediately recognizes: cover overhead, open spaces between stems, and shelter near the bottom. 2

Follow one stipe toward its holdfast, keeping clear of the plant. Then look across any neighboring bare patch. Losing the canopy also removes much of the living structure that fishes and invertebrates use. The change is visible at several levels of the same dive, from the light above you to the cover around the rocks below.

A purple urchin belongs in this habitat. Its presence does not by itself diagnose a problem. The question is how many are exposed, what they are eating, and whether kelp continues to grow around them. A visitor who counts every urchin as damage will miss the difference between a grazer living within a forest and grazing that prevents a forest from returning.

When the food supply changes, the grazers change too

In the forest, look for an urchin sheltered in a crack with a detached piece of algae within reach. Drifting kelp can deliver food to that hiding place. When less arrives, urchins may emerge to graze attached growth. A wave of sea star wasting began in 2013; the loss of sunflower sea stars removed an urchin predator, and marine heat beginning in 2014 put kelp under further strain. The outbreak followed several pressures acting together. 1 3

Now inspect the open bottom without touching it. A field of exposed urchins and little new kelp suggests a difficult restart: less forest supplies less drifting food, while hungry grazers can consume growth that might rebuild the canopy. The animals can persist in poor condition. A barren can therefore remain full of living urchins even when it has little food to offer them.

The central California story should also remain distinct from the extensive bull-kelp losses farther north. Monterey’s giant-kelp forests, northern California’s bull-kelp systems, and their predator communities overlap in some mechanisms but differ in geography and history. A statewide headline does not describe every reef in the same way.

The valuable part is inside the shell

From shore, an otter returning to the surface with prey gives you the next part of the story. It has spent energy descending, finding food, and bringing the meal back up. Its dense fur and high metabolism help it stay warm, but staying fed requires substantial daily intake. A conspicuous urchin on bare rock is useful only if the food inside repays the effort of collecting and handling it. 4

The difference appears clearly in the researchers’ photographs of opened urchins: substantial yellow tissue in a well-fed animal, much less in a starved one. That tissue is the gonad, which provides an energy-rich meal. Joshua Smith’s team measured urchin condition and paired underwater surveys with observations of otter feeding. You can study those photographs while leaving the living urchins on your dive intact. 3

The university’s account of the research describes a marked increase in urchin consumption by otters after the outbreak, yet persistent barrens. The apparent contradiction disappears once you ask which urchins the otters were eating. Well-fed animals in forest patches offered more than their starved counterparts on bare reef. 1

An otter bringing up a profitable urchin from the kelp patch also removes a grazer from that patch. Repeat the meal often enough and the remaining forest benefits. The neighboring barren may offer so little food inside each shell that it attracts far less of that hunting. Watch where the otter repeatedly dives: the location of the meal helps explain the location of the benefit.

Protection and recovery are different outcomes

At the edge of a surviving patch, look for living kelp still reaching upward. That patch holds habitat now and can supply spores to the surrounding area. Across the boundary, the return of a canopy also depends on young kelp surviving grazing. Removing pressure from an existing forest and getting new kelp established on bare rock are different challenges. 1

A separate study published in 2025 makes that point at a broader scale. Researchers compared three decades of community data from western Vancouver Island and San Nicolas Island in southern California. Kelp and urchin responses to otter recolonization followed different trajectories. Their modeling indicated that changing interactions among community members mattered, rather than a single constant effect of otters on urchins. This was not a new survey of the Monterey patches; it was evidence that the same predator can have different effects in different ecological settings. 5

This is why the boundary matters more than a simple count of otters offshore. Look for new kelp as well as mature canopy, and for exposed grazers as well as sheltered ones. Those observations help frame the local question: is a forest holding its ground, expanding, or failing to establish on neighboring rock? Otters are a major part of the answer, alongside the conditions in which the kelp must grow.

How to read the scene at Monterey

From shore, look for the relationship between surface kelp and repeated otter dives. Binoculars let you watch without drawing closer. A series of dives in one area raises a useful question about the food below; it does not let you identify the prey unless you can actually see it.

Underwater, compare the canopy, the attached growth on the rocks, and the positions of urchins. Are they mostly tucked into crevices or spread across exposed surfaces? A single dive cannot reproduce a field study, but it can teach you which observations are relevant. Do not move or break urchins to investigate their condition, and give otters room to rest and feed.

Use the Monterey Bay map, sea otter profile, and purple sea urchin profile to connect the animals with the place. Then look again at the boundary between forest and bare rock. The puzzle is not whether otters matter. It is where their feeding makes a difference, and why.